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Inconsistent Fermenter Attenuations

Hello,
WE are having problems with our fermenting beer reaching final attenuation.WE are a 25 bbl brewery with single walled open fermenters that are unfortunately hand controlled for accurate ferm. temp. We generally manage to keep the temp between 69-73 degrees, but during peak fermentation are reluctant to chill the fermenting beer by more than a degree or two since we have hade bad experiences with the fermentation sticking if the temp drops too much.Because of this we have let the ferm temp rise to the upper temp. and then let it slowly drop as the ferm. activity deceases and the single walled ferms start to cool the beer.The yeasst is wyeast 1318 Scotish Ale strain.
Our beers have been made many times on other systems using closed fermenters and our Pale Ale usually finishes out at beween .08-.10 S.G. on these systems.Currently we get final gravities of between .013-.016.Since we bottle this is causing a secondary ferm in the bottle which we have had to counter by bottling at less carbonation than we prefer, but as such are not getting routine and consistent bottle carb.
An example of this problem is the past two 25 bbl batches of pale ale that were done back to back one per day. The first was over pitched and achieved a very agressive ferm that was finished in three days. It reached a F.G.of .010 S.G.This ferm was so agressive that we did not rouse the yeast with the rousing tools during the end of the ferm. but got one of the better attenuations so far.The other beer also had a very active ferm.but we roused the yeast regularly for three days and only managed to get a .014 which has been usual so far.These beers were both brewed exactly the same way so......... why the difference.
We do have two rousing pumps that recirc. the yeast during ferm. but have not used those recently since they did not appear to help and the rousing tools appeared to do a far better job of putting the floculated yeast bck to the top and into suspension.If you have any helpful answers we would appreciate your input as the secondary ferm. problem must be solved as it hurts our product and of course sales. Cheers

Attenuation is created during mashing. If you do not have an enzyme deactivation rest after mashing, the saccharification enzymes continue to work in your kettle until the temperature rises enough to deactivate them (>170°F).

You mentioned sluggish and rapid fermentations (inconsistent results). How do you aerate your wort? Oxygen or compressed air? Do you aerate after the wort cooling process? Consistent aeration to ~10 ppm or "saturation" will help you acheive consistent fermentations and will help you reach attenuation. You can check this with a Dissolved Oxygen meter. Mettler-Toledo and Orbisphere make excellent units. With proper aeration you could discontinue "rousing".

Have you tried doing rapid attenuations in your laboratory? You can get results within 24hrs, and you can see if your mashing procedures are consistent.

In the little info you have provided, there could be multiple problems preventing you from reaching your goals. A consultant could really help you over this little bump...

It could also be a yeast issue as Wyeast 1318 is highly flocculent. Is your yeast strain fresh or getting up there in serial repitches? If yeast is not fresh the flocculation could be compounded by your yeast harvesting methods if yeast is not pulled from the middle of the yeast bed. In particular you could be selecting highly flocculent, low viability yeast if pulling the first yeast off the yeast bed. As zbrew2k mentioned it could be many different factors in your AE fluctuations such as mash program and mashing temperature control. Hope this information gives you some areas to look over.

I would vote for more oxygen. Also, try to avoid the lower temp range, as this will slow fermentation, creating less heat to counteract the cool atmosphere's effect, causing a downward spiral of temp and ferm activity (read attenuation). I have had same problem with closed single wall fermenters in cold rooms. Thermostats are really cheap compared to failed bottle product. Two to three hundred bucks for tstat and solenoid could mean both lots better profit and sleep at night. Hey, the yeast would get to finish dessert, too!

If your brewing liquor is too alkaline, your yeast will flocculate before their job is done (sometimes even with good aeration). Not alkaline enough...yeast may not floc. You compensate by monitoring your alkalinity daily, and making adjustments to neutralize to a desirable level for your yeast. You can then supplement with a source of zinc to help the fermentation finishes properly.

Thank you for your replys. More info to consider

Thanks so much for the input. Here is some more information to consider.Hot Liquor/ Strike temp is 172F( PH= 6.5-6.7 ) Spare= 172F. Mash Temp <156F.
Ph of wort = 5.1- 5.4. We use oxygen that is introduced to the chilled wort ( 68-70F ) at a rate of 3 Ltrs/ minute. We pitch the yeast once the level in the Ferm. hits the Temp. Probe to be sure. We use YEASTX from brewer's wholesale in the boil as a yeast nutrient.
If any of this data helps with yoir advice please let me know. Thanks again for you kind responses. Cheers!

Do you have any water chemistry analyses? What is your mash water Ca++ ion levels, Mg, SO4, etc...?

Remove the oxygen injector, and soak it in caustic, and rinse, and then in acid. If you have an enzyme cleaner, use that also. Then sanitize and replace. Double your oxygen injection rate and see how it affects your fermentation. You could follow the cell count in the fermentor every few hours to see how the fermentation progresses. Match that with your Balling curve (gravity drop curve), and you can help you nail this problem down.

Keep track of the fermentor temp with an accurate thermometer to ensure the temp is consistent.

Pretty sweet to have visual flow gauge on oxy inlet (McMaster-Carr or other), so even if there's no sightglass to see, you know how much is going in. Regardless, add more oxy and call back in the morning.

Do you have any yeast information such as pitching rate and verification from a haemocytometer after wort cooling? How long is your lag phase? Acid washing the yeast? I think the wort aeration may get you over the hurdle but it may not be THE solution to your AE issues. You can oversaturate wort if using pure Oxygen and you may cause excessive foaming in FV. I find it odd that you have to rouse the yeast on a regular basis. My experience with rousing is it makes no difference in final AE and can actually do some damage to beer profile by causing the yeast cells to rupture.